fix(telegram): convert markdown to HTML before splitting to avoid message length overflow

Cherry-picked from #3316 (himax12). When streaming completes in send_delta(),
the code was splitting raw markdown text by 4000, then converting to HTML.
The markdown-to-HTML conversion adds 10-33% characters, which could push
the result over Telegram's 4096 character limit.

The fix converts markdown to HTML first, then splits by 4096 (actual Telegram
limit), ensuring the edited message always fits.

Fixes #3315

tests/channels/test_telegram_channel.py

@@ -467,8 +467,45 @@ async def test_send_delta_stream_end_falls_back_on_bad_request() -> None:
 
 @pytest.mark.asyncio
 async def test_send_delta_stream_end_splits_oversized_reply() -> None:
-    """Final streamed reply exceeding Telegram limit is split into chunks."""
-    from nanobot.channels.telegram import TELEGRAM_MAX_MESSAGE_LEN
+    """Final streamed reply exceeding Telegram limit is split into chunks.
+
+    The fix converts markdown to HTML first, then splits by 4096 (actual Telegram
+    limit), ensuring the edited message always fits within Telegram's constraint.
+    Previously, the code split by 4000 (TELEGRAM_MAX_MESSAGE_LEN) before HTML
+    conversion, which could still overflow when HTML tags were added.
+    """
+    channel = TelegramChannel(
+        TelegramConfig(enabled=True, token="123:abc", allow_from=["*"]),
+        MessageBus(),
+    )
+    channel._app = _FakeApp(lambda: None)
+    channel._app.bot.edit_message_text = AsyncMock()
+    channel._app.bot.send_message = AsyncMock(return_value=SimpleNamespace(message_id=99))
+
+    oversized = "x" * (4000 + 500)
+    channel._stream_bufs["123"] = _StreamBuf(text=oversized, message_id=7, last_edit=0.0)
+
+    await channel.send_delta("123", "", {"_stream_end": True})
+
+    channel._app.bot.edit_message_text.assert_called_once()
+    edit_text = channel._app.bot.edit_message_text.call_args.kwargs.get("text", "")
+    assert len(edit_text) <= 4096, f"edit_text length {len(edit_text)} exceeds Telegram's 4096 limit"
+
+    channel._app.bot.send_message.assert_called_once()
+    send_text = channel._app.bot.send_message.call_args.kwargs.get("text", "")
+    assert len(send_text) <= 4096
+    assert "123" not in channel._stream_bufs
+
+
+async def test_send_delta_stream_end_html_expansion_does_not_overflow() -> None:
+    """Markdown that expands when converted to HTML is still split correctly.
+
+    This is the actual bug from issue #3315: markdown like **bold** expands to
+    <b>bold</b>, adding ~33% characters. A 3600-char message with heavy markdown
+    could become 4800+ chars after HTML conversion, exceeding 4096 limit.
+    The fix converts to HTML first, THEN splits by 4096.
+    """
+    from nanobot.channels.telegram import _markdown_to_telegram_html
 
     channel = TelegramChannel(
         TelegramConfig(enabled=True, token="123:abc", allow_from=["*"]),
@@ -478,14 +515,21 @@ async def test_send_delta_stream_end_splits_oversized_reply() -> None:
     channel._app.bot.edit_message_text = AsyncMock()
     channel._app.bot.send_message = AsyncMock(return_value=SimpleNamespace(message_id=99))
 
-    oversized = "x" * (TELEGRAM_MAX_MESSAGE_LEN + 500)
-    channel._stream_bufs["123"] = _StreamBuf(text=oversized, message_id=7, last_edit=0.0)
+    markdown_text = "**bold** " * 400  # 3600 chars raw, expands ~33% to 4800 HTML
+    raw_len = len(markdown_text)
+    html_len = len(_markdown_to_telegram_html(markdown_text))
+    assert html_len > 4096, f"Test precondition failed: HTML should exceed 4096 (was {html_len})"
+
+    channel._stream_bufs["123"] = _StreamBuf(text=markdown_text, message_id=7, last_edit=0.0)
 
     await channel.send_delta("123", "", {"_stream_end": True})
 
     channel._app.bot.edit_message_text.assert_called_once()
     edit_text = channel._app.bot.edit_message_text.call_args.kwargs.get("text", "")
-    assert len(edit_text) <= TELEGRAM_MAX_MESSAGE_LEN
+    assert len(edit_text) <= 4096, (
+        f"HTML text length {len(edit_text)} exceeds Telegram's 4096 limit. "
+        f"Raw was {raw_len}, HTML was {html_len}."
+    )
 
     channel._app.bot.send_message.assert_called_once()
     assert "123" not in channel._stream_bufs